Various substances have been developed in the past for use as coatings and films to be applied to metal, glass, and ceramic substrates by simple commercial processes such as spraying, dipping, or spinning. Typically, such substances developed for use as coating and films were organic polymer based coatings. While some organic polymer based materials found some degree of commercial acceptance, many of them had very limited performances in hostile environments, e.g. high temperature, corrosive, or abrasive environments.
In recent years there has been a continuing interest in developing silicon-based polymeric coatings, i.e. glass-like coatings, as substitutes for organic-polymer based ones. Ideally, such a coating should be refractory, corrosion resistant, electrically insulating, flexible enough to be coated on a wire, and also be applicable at low temperatures compatible with the desired substrate.
Typically, these silicon basec coatings are made by first dissolving silicon alkoxide species represented by the general formula Si(OR).sub.4, wherein R typically represents an alkyl radical, in an alcohol solvent and then at least partially hydrolyzing the silicon alkoxide in the presence of water, usually in a stoichiometric molar excess amount. The hydrolysis can be typically represented by the following equation: ##STR1## Thus, according to equation (1) above, by the standard method one achieves predominantly a tetrahydroxy-substituted silicon molecule. These partially hydrolyzed silicon alkoxide molecules are then polymerized to form a glass-like network linked by bridging oxygen atoms.
Dilute solutions of the polymerized network (2-5 equivalent wt % oxides) can be applied to various substrates by dipping, spinning, and drying operations. When applied to a substrate, the glass-like polymers react chemically with the surface and thus cause complete wetting and subsequent adherence to the surface of the substrate. The microporous silica-like glass film can be converted to a dense like glass film by heating the coated substrate at a temperature of about 60.degree. C. for at least 12 hours.
Even though the use of an alcohol is required in the silicon polymer formation and subsequent dilution steps, a difficulty arises from its use because of the fact that the presence of alcohol lowers the vapor pressure of the silicon polymer solution. This is because the low vapor pressure causes the rapid evaporation of the silicon solution during the drying process after it is applied as a coating to a substrate. The rapid evaporation causes the glass-like silicon polymer to lose its effectiveness as a coating and also causes the formation of voids and cracks in the coating due to the drying stresses generated by the evaporation of volatiles or other low molecular weight intermediate compounds from gel micropores in the polymer.
What is needed is a process for making and applying a polymerized silicon coating which prevents or eliminates the problem of rapid evaporation of the coating during its drying.